Laser-Ablation Deposition and Characterization of Ferroelectric Capacitors for Nonvolatile Memories
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MRS BULLETIN/JUNE 1996
pacitors was hampered in previous decades by one or a combination of problems related to either the reliable performance of the PZT ferroelectric capacitor or to the growth and processing of capacitor layers. The worldwide work of the last seven years demonstrated that the interfaces between the top and bottom electrodes and the ferroelectric layer play critical roles in determining the properties of PZT-based ferroelectric capacitors. Materials-integration strategies developed during the last five years resulted in the control of various electrical properties of PZT-based capacitors, which are needed to produce commercial NVFRAMs: (a) Near elminination of polarization fatigue (i.e., the loss of switchable polarization of a capacitor subjected to repeated bipolar voltage pulses) by controlling the PZT/electrode interface. Fatigue in PZT-based capacitors has been mainly observed when using pure metallic electrodes. It has been linked to a major extent to phenomena occurring at the bottom and/or top electrode-ferroelectric interfaces. These phenomena include the presence of oxygen vacancies and their action as charge-trapping centers. In addition, internal interfaces in the PZT layer such as domain walls and grain boundaries, and microstructural
defects such as oxygen vacancies, in conjunction with charge trapping, can influence ferroelectric properties—for example, fatigue. Work during the past three years has demonstrated that polarization fatigue in PZT-based capacitors can be overcome for all practical purposes by replacing metallic Pt electrodes with metal-oxide electrodes such as RuO22'3 or with any of the perovskite metal oxides such as YBa2Cu3O6 + , (YBC)4 and Lao.5Sro.5CoO, (LSC),5" or with hybrid metal-oxide electrodes,7"9 among others, (b) Polarization retention (i.e., the time in which a certain state of polarization is retained in a PZT-based capacitor) of several years has been achieved in PZT-based capacitors with the oxide electrodes, (c) Negligible imprint (i.e., the tendency of a capacitor to revert to a prior state of polarization or memory state after being switched to the opposite state) has been achieved in PZTbased capacitors using the oxide electrode, (d) Low leakage currents (in the range of l(T 8 -l(r 9 A/cm2), needed for a commercial memory, have been obtained in PZT capacitors with hybrid Pt-oxide electrodes. An alternative ferroelectric material that has emerged as a forerunner during the last five years together with PZT for application to NVFRAM is SrBi2Ta2Oi) (SBT), which pertains to a family of layered perovskites.10 A distinctive feature of the SBT ferroelectric material is that capacitors with controlled properties such as those just described for PZTbased capacitors can be fabricated using elemental Pt electrodes." However, the maximum polarization hitherto obtainable with SBT-based capacitors is substantially lower (AP ~ 20 juC/cm2)11 than that characteristic of highly oriented PZTbased capacitors (AP ~ 45-50 /aC/cm2).8 Several issues need to be considered
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